Oil is removed from the ground using a pump-jack. This equipment is mounted on the surface of the earth above an oil reservoir. The pump-jack is connected to a down-hole pump located at the bottom of an oil well by a string of interconnected sucker rods, which extends inside a production tubing. Through the action of the pump-jack, oil is pumped from the reservoir to the surface for collection.
A sucker rod is typically a rod having a length of between 24 and 32 feet and a diameter of 0.5 to 2 inches. During pumping, the string of sucker rods performs a reciprocating or alternative movement, which may produce deflections of the string. The sucker rods are thereby subjected to wear due to frictional contact with the inner wall of the production tubing. Even though the fluid environment serves as a lubricant, abrasion does occur over the surface of the sucker rods. Additionally, tools used for assembling sucker rods into a string may also cause tearing of the rod surface.
In some oil wells, the fluid includes dissolved salts and undissolved minerals which may have an additional abrasive or corrosive effect on the sucker rod surface. The presence of hydrogen sulfide (H2S), sulfurs (HS, S), water, salty water, hydrogen ions, CO2 in aqueous solution, and other corrosive chemical compounds, also weaken the sucker rods structure, thereby reducing their fatigue limit. When the attack is particularly harsh, sucker rods break.
When a rod fails, the whole sucker rod string needs to be pulled from the well and inspected, and defective rods must be replaced. This procedure increases costs when it becomes frequent. Additional costs related to corrosion problems result in losses in production, added costs for new materials, and increased pulling costs.
Therefore, it is desirable to provide a sucker rod capable of reducing friction and/or resisting corrosion in environments like those found in oil wells.